This invention relates to a circuit for reading charges and a process for reading charges.
The invention also relates to a radiation detector circuit.
According to known art, a radiation detector circuit comprises a set of elementary detectors to convert detected radiation into electrical charges and a circuit for reading the detected charges.
The circuit for reading detector charges comprises an elementary integrator circuit of the switched follower type, for each elementary detector. An elementary integrator circuit is shown in FIG. 1. It comprises an integration capacitor Cint, a switch Ia and a follower circuit S. Detected charges form a current Iph that is integrated in the capacitor Cint. The switch Ia is used to reinitialize the potential of the read node by application of a reinitialization voltage Vr. The follower circuit S outputs a voltage variation Vs proportional to the current Iph.
The maximum allowable photonic current Iphmax, the integration time Ti of charges in the capacitor Cint and the integration capacitor Cint satisfy the following relation:             Iph      max        xc3x97    Ti    ≤                    Δ        ⁢                  xe2x80x83                ⁢                  V          S                ⁢        max            G        xc3x97          C      int      
where xcex94Vsmax is the maximum voltage excursion at the output from the follower S and G is the voltage gain of the follower. In the rest of this description, the gain G will be assumed to be equal to 1, which is the theoretical value of the gain of a voltage follower.
Similarly, the minimum allowable photonic current Iphmin due to the photonic noise charge (read noise) satisfies the following relation:
{square root over (qxc3x97Iphminxc3x97Ti)}=Cintxc3x97 less than Vbxe2x88x92pel greater than 
where q is the charge of the electron and  less than Vbxe2x88x92pel greater than  is the rms noise at the input to the follower.
These two relations may be combined to obtain a current input excursion in the elementary integrator circuit:             Iph      min              Iph      max        =                    1                  q          xc3x97          Δ          ⁢                      xe2x80x83                    ⁢                      V                          S              ⁢                              xe2x80x83                            ⁢              max                                          xc3x97              C        int            xc3x97         less than           Vb      -      pel        ⁢           greater than       2      
For example, for xcex94Vsmax equal to approximately 2 volts, Cint equal to approximately 1 pF and  less than Vbxe2x88x92pel greater than  equal to approximately 100 xcexcVxe2x88x92rms, the input excursion is equal to approximately {fraction (1/32)}, knowing that the signal to noise ratio of the photodiode is degraded by 2 under the minimal illumination.
These performances are sufficient for some applications. However, other applications necessitate an increase in the input excursion of the elementary point to enable a wider illumination range, for example to solve glare problems or problems with scenes with high contrast.
The invention provides a good solution to this need.
The invention relates to a circuit for reading charges comprising a capacitive means with capacitance Ca to store charges and means of reading a voltage sampled at the terminals of the capacitive means. The capacitive means is built up from a first capacitive means and a second capacitive means with a capacitance Cb less than Ca. The read circuit comprises means of comparing the voltage read at the terminals of the capacitive means with a threshold value, means of controlling the transfer of charges stored in the first capacitive means to the second capacitive means with a capacitance Cb less than Ca when the voltage read at the terminals of the capacitive means exceeds the threshold value and means of reading a voltage sampled at the terminals of the second capacitive means.
The invention also relates to a process for reading charges comprising a step to integrate charges in a capacitive means with capacitance Ca and a step to read a voltage at the terminals of the capacitive means. The capacitive means consists of a first capacitive means and a second capacitive means with capacitance Cb less than Ca. The process comprises firstly a step to compare the voltage read at the terminals of the capacitive means with a threshold voltage, a step to transfer charges stored in the first capacitive means to the second capacitive means with a capacitance Cb less than Ca when the voltage read at the terminals of the capacitive means exceed the threshold value, and a step to read a voltage at the terminals of the second capacitive means.
The invention also relates to a radiation detector circuit comprising at least one photo detector to convert detected radiation into electric charges and a circuit to read electric charges. The read electric charges circuit is a circuit to read charges according to the invention.